Description
The Network Slice Admission Control Function (NSACF) is a critical control plane network function introduced in 3GPP Release 17 as the central entity responsible for executing Network Slice Admission Control (NSAC) policies. It is a logical function that can be deployed as a standalone network element or collocated with other control plane functions. The primary role of the NSACF is to manage and enforce the maximum allowed number of User Equipments (UEs) registered to each network slice instance (NSI). It does this by maintaining accurate, real-time counters for UE registrations and deregistrations on a per-slice basis. The AMF, which handles UE registration requests, interacts with the NSACF to determine if a UE can be admitted to a requested slice.
The NSACF operates through a set of service-based interfaces, primarily exposing services to the AMF. When a UE initiates a registration procedure including a Network Slice Selection Assistance Information (NSSAI), the AMF invokes a service operation on the NSACF (e.g., Nnsacf_NSAC_Control service) to request an admission check. The NSACF receives this request, which includes the slice identifier (S-NSSAI), and checks its internal database or policy repository for the configured maximum UE limit and the current registration count for that slice. Based on this check, it returns an admission control decision (allow or reject) to the AMF. If allowed, the NSACF increments its internal counter. Conversely, when a UE deregisters or its registration context is released, the AMF notifies the NSACF, which then decrements the corresponding counter.
Architecturally, the NSACF is a stateful function that must ensure data consistency and reliability. It may interact with a Unified Data Repository (UDR) to persistently store slice admission control policy data and potentially the registration counts for recovery purposes. The NSACF's design also considers scalability and high availability, as it becomes a central point for slice admission control across the entire network. Its functionality is tightly integrated with the overall network slicing management system, including the Network Slice Selection Function (NSSF) and the Policy Control Function (PCF), to ensure a cohesive slice management and policy enforcement framework.
Purpose & Motivation
The NSACF was created to provide a dedicated, scalable, and centralized point for enforcing Network Slice Admission Control policies. Prior to its introduction, there was no standardized function to manage the scale of network slices in terms of connected devices. Relying on individual network functions like the AMF to locally enforce such limits would be inefficient, inconsistent, and difficult to manage network-wide. The NSACF solves this by centralizing the logic and state for slice admission, ensuring a single source of truth for slice registration counts across all AMFs in the network.
This centralization is crucial for several reasons. First, it guarantees consistent policy enforcement regardless of which AMF a UE attaches to, which is essential in a cloud-native, distributed core network. Second, it simplifies operations and policy management for network operators, as they can configure and update slice capacity limits in one logical function. Third, it enables advanced features like dynamic capacity adjustments and integration with network analytics. The NSACF's creation was a direct response to operator requirements for robust commercial slicing, where guaranteeing a slice is not oversubscribed is as important as guaranteeing its performance, making it a cornerstone for slice-as-a-service business models.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (580 CRs across 6 releases). Complements the general historical overview above with the evidence-based evolution of this function.
In Release 15, the NSACF was newly introduced as a dedicated function to manage network slice admission control, as formally specified in TS 29.536. Its introduction provided the foundation for slice-based congestion control, including S-NSSAI based congestion control, and enabled the network to manage the modification of the set of network slices for a UE. This function was central to implementing slice availability updates and admission policies for network slices within the 5G core.
- Network control for always-on PDU sessions TS 24.501CR0107
- Clarification on congestion control upon intersystem change TS 24.501CR0604
- Control of the Messages triggering Paging at AMF TS 23.501CR0033
- Slice selection cleanup TS 23.501CR0047
- NAS congestion control update TS 23.501CR0051
- Complete of IMS Emergency support in 5G including slice and local numbers TS 23.501CR0052
+ 64 more changes
In Release 16, the enhancements for the NSACF introduced specific control mechanisms for network slice management, including the new Slice-Specific Authentication and Authorisation capability. Furthermore, the release added procedures for coordinated resource optimization across multiple slices sharing common network resources, such as between PNI-NPN and PLMN slices. These additions provided more granular policy control and lifecycle management for network slices based on vertical application requirements.
- Introducing Rate Control for 5G CIoT TS 23.501CR0752
- Introduction of data transfer in Control Plane CIoT 5GS Optimisation TS 23.501CR0889
- CIoT Introduction of Overload Control TS 23.501CR0894
- Introduction of Service Gap Control TS 23.501CR1014
- UL CL/BP controlled by I-SMF TS 23.501CR0848
- Enhancement on slice interworking--501 TS 23.501CR0850
+ 128 more changes
In Release 17, the NSACF was enhanced with new capabilities including support for Network Slice Admission Control in the EPC, roaming support for NSAC in the VPLMN, and admission control with consideration of access type. It also introduced support for Emergency and Priority Services within NSAC and the ability to use NWDAF analytics for network slice restriction. Furthermore, Release 17 added support for the GSMA NG.116 attributes "Simultaneous Use of a Network Slice" and "Maximum DL/UL throughput per slice/UE."
- Support of different slices over different Non 3GPP access TS 23.501CR2525
- Network Slice restriction based on NWDAF analytics TS 23.501CR2567
- TS23.501 KI#1 Network Slice Admission Control Function (NSACF) definition TS 23.501CR2679
- TS23.501 KI#2 Network Slice Admission Control Function (NSACF) definition TS 23.501CR2680
- Support for UE-Slice-MBR TS 23.501CR2706
- Adding the usage of Session Management Congestion Control Experience analytics TS 23.501CR2708
+ 113 more changes
In Release 18, the NSACF was enhanced with a hierarchical architecture to support centralized control within a PLMN across multiple service areas and to provide EPS counting. It also introduced support for network slice replacement during handover, graceful termination of PDU sessions during slice decommissioning, and improved network control of UE behavior for a slice.
- Multiple NSACF architecture enhancement TS 23.501CR3785
- KI#4: Support for Centralized NSACF in a PLMN with multi-service areas TS 23.501CR3822
- KI#4: Support for HPLMN admission mode while Roaming TS 23.501CR3823
- Policy control enhancements to support multi-modal flows TS 23.501CR3864
- Change of Network Slice instance for PDU sessions TS 23.501CR3867
- Improved network control of the UE beahviour for a network slice TS 23.501CR3939
+ 161 more changes
In Release 19, the NSACF function was enhanced with new capabilities for slice requirement verification and alignment, as well as for supporting network slice allocation in an NSaaS model. It also introduced support for network slice change based on an Application Function (AF) request. Furthermore, Release 19 added support for providing energy-related information per network slice.
- Enhancements to Network slice allocation procedure in NSaaS model TS 23.435CR0004
- Enhancements to Slice requirement verification and alignment capability TS 23.435CR0005
- Enhancements to Slice requirement verification and alignment capability TS 23.435CR0017
- Enhancements to Slice requirement verification and alignment capability TS 23.435CR0022
- Control Plane and User Plane Protocol stacks involving the MWAB node TS 23.501CR5561
- KI#2: UE subscription and policy control for energy efficiency and energy saving TS 23.501CR5739
+ 82 more changes
In Release 20, the NSACF function was enhanced to support policy control for network energy saving, including the exposure of energy consumption information. This was part of a broader update to the architecture and exposure interface functions to enable these new energy-saving policy mechanisms.
Explore further
Broader topics and technologies where NSACF plays a role.
Defining Specifications
3GPP specifications that define or reference NSACF, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 23.435 vj30 | Network Slice Capability Exposure Procedures | Rel-19 |
| TS 23.501 vk00 | 5G System Architecture Stage 2 | Rel-20 |
| TS 23.700 vk00 | XR Services Application Enablement Layer | Rel-20 |
| TS 24.501 vj50 | 5G NAS Protocols Specification | Rel-19 |
| TS 26.804 vj10 | 5G Media Streaming Extensions Study | Rel-19 |
| TR 26.942 vj00 | Study on Media Energy Consumption Exposure & Evaluation | Rel-19 |
| TS 28.203 vi10 | Charging management | Rel-18 |
| TR 28.843 vi10 | Technical Report on Charging Aspects for Vertical Scenarios | Rel-18 |
| TS 28.879 vj10 | OAM for Service Management Exposure Study | Rel-19 |
| TS 29.522 vj40 | 5G NEF Northbound APIs Stage 3 | Rel-19 |
| TS 29.536 vj30 | NSACF Service Based Interface Protocol | Rel-19 |
| TS 29.552 vj40 | 5G Network Data Analytics Signalling Flows | Rel-19 |
| TS 29.574 vj40 | 5G Data Collection Coordination Services Stage 3 | Rel-19 |
| TS 29.575 vj40 | 5G Analytics Data Repository Services Stage 3 | Rel-19 |
| TS 32.240 vj40 | Charging Management Architecture & Principles | Rel-19 |
| TS 32.290 vj50 | 5G Charging for Service Based Interface | Rel-19 |
| TS 32.291 vj40 | Charging Management: Service-Based Interface Protocol | Rel-19 |